Pulse stretching and compression is useful in many applications. One of the most common techniques to achieve efficient pulse stretching/compressing is to use a long dispersive optical fiber within which the light pulse is guided and propagates. A key attribute of using optical fiber to achieve this task is the chromatic dispersion of the optical fiber (typically silica glass), in which different frequency components experience different refractive indices and effectively travel at different speeds along the fiber. It thus separates different frequency components within the pulse in time, i.e. pulse stretching. The dispersive elements can also be used as a compressor of optical pulses in which a temporally spread optical pulse can be recompressed into a pulse that satisfies required applications. For example, this can be done to avoid excessive temporal broadening of ultrashort pulses, which can cause the distortion of signals in telecommunications and in optical microscopy/imaging.
Although optical fibers have been well recognized as one of the most conventional dispersive elements (for pulse stretching/compression), they have limitations which have impeded the utility of optical fibers in a wider range of applications. The allowable operation wavelength range of pulse stretching/compression is restricted by the material (more specifically, the optical loss) of the optical fiber. As the common optical fibers are made of silica glass, the optical loss and thus efficient pulse stretching/compression is optimized to the near infrared spectral windows from 1 μm to 1.5 μm. This means that the wavelength of the optical pulses on which optical stretching/compression can be performed strongly depends on the material loss of the optical fiber, ultimately limiting the operational range of wavelengths.
Also, in optical fibers, the pulse stretching/compression cannot be actively or dynamically tunable. The amount of pulse stretching/compression using optical fibers is governed by the dispersion that optical pulses experience. The total dispersion is directly proportional to the fiber length, which is typically fixed once the fiber is fabricated and not flexibly (and widely) adjustable.
In addition, the group delay dispersion (GDD) accumulated by optical pulses in optical fibers is limited, and the amount of temporal stretching also depends on the optical bandwidth of the pulses. The consequence is the need for a long fiber (on the order of 10's of km of standard telecommunication fiber) in order to attain enough GDD for efficient pulse stretching/compression. From the design point of view, using optical fibers for pulse stretching/compression is spatially inefficient.
Moreover, optical nonlinearity that comes with all materials in existing devices is unavoidable and detrimental for pulse stretching/compression. Especially in optical fibers, the optical pulses not only experience linear dispersion (i.e., constant refractive index at each frequency), but also the nonlinear effect in which the refractive indices depend on power distribution of the optical pulse envelope. The optical pulses and therefore the encoded information are eventually distorted during the stretching/compression process.
On the other hand, pulse stretching has been used together with a technique performing time-to-space mapping to achieve optical beam scanning or steering. This approach allows optical beam scanning with the need for mechanical moving parts, such as scanning mirrors, and thus bypasses the fundamental speed limitation (limited by inertia), as well as the motion artifact of such mechanical-based beam scanners. The applications of laser beam scanning have been widely covering from barcode scanning, biomedical imaging, material science research, laser beam machining and ablation, and automated surface inspection in manufacturing industries (including semiconductor integrated circuit (IC) chip manufacturing in the very-large-scale integration (VLSI) industry). In these applications, optical beam scanning is done by spatially deflecting the beam using optical elements. Common choices include galvanometric mirrors and acousto-optic deflectors.
Generally, beam scanning can be classified into active or passive scanning depending on the practical implementations. Active beam scanners require a controllable element to alternate (or steer) the direction of an optical beam. For example, in laser scanning imaging/microscopy (widely adopted in life science or material science applications), a laser beam can be angularly steered by a galvanometer-mirror continuously within a certain range of angles. Combining with a proper relay lens system, such angular beam displacement can be transformed into lateral beam displacement such that a focused beam can be laterally scanned across the sample under test (e.g., biological cells/tissues). The spatial information of the specimen (due to absorption, scattering, or luminescence) is temporally read-out by the scanned beam. Thus, the target image can be retrieved from the serial-time signal using a single pixel photodetector. Ultimately, the scanning rate of these techniques is fundamentally limited by the speed of the mobilized deflecting optics elements and the mechanical movement of these devices. Galvanometric mirrors are widespread in most commercial laser scanning systems; however, because of the mechanical inertia in all galvanometric mirrors, including microelectromechanical system (MEMS) scanners, it only can provide a one-dimensional (1-D) line-scanning rate up to 500 Hz or 1 kHz. Modest improvement in scan speed can be achieved by operating the mirrors at their resonant frequencies (i.e., resonant galvanometric mirrors)—mostly up to about 10 kHz. To overcome the mechanical limitation, both acousto-optic (AO) and electro-optics (EO) modulators have been invented to achieve higher scanning rates on the order of sub-MHz to MHz. However, the high scan speed is achieved with these devices at the expense of a smaller range of the scanning angle and number of resolvable scanned points (i.e., field of view). AO devices also suffer additional optical loss due to the diffraction effect of the device whereas EO devices typically require high voltage (>100 V) to achieve reasonable a scanning range for practical imaging applications.
In contrast to active beam scanning, passive beam scanning is a technique that does not involve direct manipulation beam steering/scanning. A notable example is beam scanning based on a spectral-encoding mechanism. In this technique, the wavelength-tunable light source (called a swept-source) with broadband wavelength spectrum is employed. The output light wavelength is swept in time. Hence, by using an optical element called a spatial disperser (e.g., a prism, diffraction grating, virtually imaged phase array, etc.), the beam can be mapped at different wavelengths to different spatial coordinates (can be in 1-D or 2-D) on the specimen under test. As the wavelength is swept in time, the beam can essentially be scanned across the specimen. The beam steering is thus achieved indirectly by wavelength tuning together with the spectral-encoding concept (i.e., wavelength-to-space mapping). The beam scanning speed of this technique is primarily determined by the wavelength-swept rate of the laser, which is typically limited to 1 kHz or 100 kHz in the current state of the art.
A conventional dispersive medium used in the optical time stretch technique is an optical fiber. For high-resolution beam scanning, a large amount of dispersion (i.e., pulse stretching) is required. This demands for a long length of fiber (>10 km) which inevitably introduces prohibitively high optical loss. The low-loss spectral region of the typical optical (glass) fiber is from about 1 μm to about 1.5 μm, limiting the operation wavelength range and thus the applications of this technique.